Sitting in the jump seat on an airliner for landing. I asked once when I was sitting upstairs on a 747 (back in the days when work paid for such things when flying transatlantic, which was of course also before 9/11), got told very apologetically that they'd got an extra crew member on board who was sitting there, but just ask again next time...

I've been lucky enough to have done this on a sabena f27 into Brussels. It was as exciting as it was terrifying. Exciting for the obvious reasons, terrifying because you get to see how rickety, flimsy and generally knackered civil aircraft are: PF's instruments had all frozen up so he was using the NFP's gauges; landing checklist was a series of hinged flaps with different tasks written on them. PF would just flip easy one down as they were completed, except in the turbulence they would randomly fall down of their own accord meaning the pilot would have to flip them back up; can distinctly remember the gear down indicators didn't.